The invention relates to a method for stabilizing a photovoltaic silicon solar cell and to a device for stabilizing a photovoltaic silicon solar cell.
Photovoltaic silicon solar cells serve for converting the energy of incident radiation into electrical energy. One crucial parameter for the economic viability of such solar cells is the efficiency, in particular, which is considerably dependent on the material quality and thus in particular on a recombination activity with regard to the generated charge carriers. Furthermore, the material costs make up a considerable proportion of the total costs of typical silicon solar cells, such that there is a need for cost-effective materials which nevertheless enable a high quality and in particular high charge carrier lifetimes. Particularly in the case of solar cells based on crystalline silicon substrates, however, degradation effects are established, i.e. the quality of the material used decreases in the course of use of the solar cell.
Therefore, there is a need for methods which stabilize such solar cells in a regenerated state with a high quality, such that no or only significantly reduced degradation takes place during use. Since the costs of the production method and thus in particular the required process time also significantly influence the total costs, there is furthermore a need to accelerate such methods.
A degradation behavior as described above was particularly pronounced in connection with boron-oxygen based defects in the semiconductor substrate of the silicon solar cell. By way of thermal treatment and typically a passivation using hydrogen, it was possible to demonstrate methods enabling such solar cells to be stabilized:
An explanatory model here is the—state model shown in FIG. 1. This takes as a basis an annealed state A, in which the solar cell, although it has a high quality, degrades during use due to being subjected to radiation, typically sunlight. In a degraded state D, the solar cell, although stable, has a significantly reduced efficiency. The aim is to obtain a regenerated state R, which is stable under typical use conditions and has an improved efficiency by comparison with the state D.
According to current knowledge, the annealed and degraded states are reversible. Under unfavorable conditions a transition from a regenerated state into the annealed state can take place, but not a direct transition into the degraded state. Furthermore, FIG. 1 makes it clear with the aid of the arrows that a direct transition from an annealed state into a regenerated state is not possible, rather a path via a degraded state must be chosen.
These effects were observed in particular with the use of Czochralski (Cz) silicon. Such effects or comparable effects likewise appear to occur in the case of multicrystalline silicon wafers as well.
Methods for stabilizing a photovoltaic silicon solar cell as described above are known from WO 2014/206504 A1, EP 1 997 157 B1 and EP 2 164 114 B1.